In recent years, to improve performance and reduce a manufacturing cost of a semiconductor device such as a solar cell or an MOS (Metal Oxide Semiconductor) transistor, an increase in diameter of a wafer formed of, e.g., silicon that is used as a substrate has advanced. Therefore, a single-crystal ingot which is grown by, e.g., the CZ method and has a diameter of 200 mm (8 inches), a diameter of 300 mm (12 inches), or more is manufactured, and an increase in diameter and an increase in weight advance.
Such a single-crystal ingot is manufactured by, e.g., a single-crystal manufacturing apparatus depicted in FIG. 2. FIG. 2 is a schematic view showing a general single-crystal manufacturing apparatus used in the CZ method.
This general single-crystal manufacturing apparatus 20 is an apparatus for growing a single crystal 31 from a raw material melt 30 based on the CZ method, and it is configured to accommodate in a main chamber 21 a crucible 23 that contains the raw material melt 30 obtained by melting a polycrystalline raw material, a heater 25 provided around the crucible 23, and a heat insulating material 26 provided around the heater 25.
In particular, components like the crucible 23, the heater 25 and the heat insulating material 26 that are heated are called hot zone components.
A pull chamber 22 configured to accommodate and take out the pulled single crystal 31 is connected to an upper end of the main chamber 21. Further, a gate valve 28 that opens/closes an opening portion at the upper end of the main chamber 21 is provided between the upper end portion of the main chamber 21 and the pull chamber 22. Furthermore, a single-crystal pulling mechanism (not shown) configured to wind up a wire 34 having a seed holder 33 attached at an end thereof is provided above the pull chamber 22.
To manufacture the single crystal 31 by using such a single-crystal manufacturing apparatus 20, a seed crystal 32 is held at an end of the seed holder 33, and the seed crystal 32 is immersed in the raw material melt 30 and gently pulled upwardly while rotating, thereby growing the rod-like single crystal 31.
At this time, an inert gas such as Ar is circulated in the chamber while performing vacuum evacuation to discharge an oxide vaporized from a melt surface.
When pulling the single crystal is finished, the heater is turned off, the gate valve is closed, and the single crystal accommodated in the pull chamber is cooled and taken out. Further, after the hot zone components are cooled down, a pressure in the chamber is restored to a normal pressure, and then the hot zone components in the main chamber are dismantled. When the dismantlement of the hot zone components is finished, cleaning, replacement and others are carried out, then the hot zone components are again assembled, charging the raw material, assembling the chambers, and melting a polycrystalline raw material are carried out, and a single crystal is again pulled up.
In such manufacture of the single crystal based on the CZ method, to achieve an improvement in productivity and a reduction in cost, an increase in a single-crystal growth rate is regarded as one large means to make many improvements in conventional examples. For example, in Japanese Patent Laid-open (Kokai) No.2000-344592 and Japanese Patent Laid-open (Kokai) No.2002-121096, there is a suggestion of placing a cooling body or a cooling pipe so as to surround the vicinity of the single crystal and cooling high temperature parts of the single crystal during the step of pulling the single crystal in order to increase the single crystal growth rate.
However, an operation cycle of manufacture of the single crystal based on the CZ method includes the step of pulling the single crystal and the above many steps other than the pulling, and further greatly reducing a pulling time is difficult under existing circumstances. Therefore, it is considered that reducing a process time for the steps other than the pulling of the single crystal is effective for improving operation efficiency, i.e., for improving an operating rate of the single-crystal manufacturing apparatus to raise productivity.
Except for the single crystal pulling step, proportions of a charging and melting time of the polycrystalline raw material before pulling the single crystal and a cooling time of the hot zone components are high.
The cooling time of the hot zone components is determined based on a condition that carbon members such as the heater are cooled down so that they are not degraded even though they are in contact with oxygen in air when restoring the pressure in the main chamber to a normal pressure. Even in case of manufacture of a single crystal having a diameter of 200 mm (8 inches) and a straight body length of 1 m which is the current mainstream, this cooling time reaches approximately 7 hours in natural cooling, and it takes a little less than a half of the process time of the steps other than the pulling.
Moreover, the charging and melting time of the polycrystalline raw material required for such manufacture of the single crystal reaches approximately 14 hours.
The melting of the polycrystalline raw material and cooling of the hot zone components are precisely an idle period of the single-crystal manufacturing apparatus. Therefore, the process time of the steps other than the pulling of the single crystal eventually becomes a cause of a considerable reduction in the operating rate of the single-crystal manufacturing apparatus. In recent years, a demand for an increase in diameter of the single crystal shows no sign of slowing down, and manufacture of a large single crystal having a diameter of 300 mm (12 inches) or more is often required. In this case, a heat capacity of each hot zone component is more greatly increased beyond a current capacity, and the cooling time is accordingly prolonged. Besides, the melting time is also prolonged in proportion to an increase in a charging amount (a melting amount) of the polycrystalline raw material. Now, a reduction in the operating rate of the apparatus due to the prolongation of the cooling time, the melting time and the like becomes a more serious problem.
In view of this, in order to reduce the melting time of the polycrystalline raw material, there is a suggestion of arranging an auxiliary heating device with a lamp or a laser separately from the heater placed around the crucible in Japanese Patent Laid-open (Kokai) No.H10-81595. In Japanese Patent Laid-open (Kokai) No.H11-255593, there is a suggestion of placing, above the main chamber, an auxiliary heating chamber having an infrared lamp located at a focal position of a reflecting plate of a paraboloidal surface.
In order to reduce the cooling time of the hot zone components, there is a suggestion of circulating an inert gas at room temperature or less into the main chamber in Japanese Patent Laid-open (Kokai) No.H9-235173.
However, in spite of an attempt to reduce the time of each step other than the pulling, a total time for manufacture of one single crystal substantially becomes longer due to the increase in size of the single crystal to be manufactured, and a drastic review has been required with regard to an operation of the single-crystal manufacturing apparatus.